This invention relates to high pressure oxygen furnaces having utility for the synthesis of high temperature superconductors.
There are two general types of high pressure furnace systems capable of heating materials in the presence of high pressure oxygen. The first furnace system is the "internally heated" system which entails placing a furnace system inside a pressure vessel. The drawbacks to this design are that the pressure vessel must be large in order to accommodate the sample, and also the thermal insulation and the electrical heating element, and the necessary heating element is prohibitively expensive. This is so since platinum is the only metal that can withstand high pressure oxygen at high temperature.
First, the larger the volume of the pressure vessel, the more oxygen will be contained, and pressurized oxygen is hazardous. Second, the large pressure vessel must withstand high pressures. Large vessels for withstanding high pressures are quite expensive.
One advantage of this type of construction is that the system can be used at very high pressures and temperatures. This high temperature and high pressure is possible because the heated region is separate and inside the pressure containment vessel. The sample and heating element, which achieves and contains the high temperatures, are surrounded by insulation inside of the vessel. The pressure containing vessel is not exposed to high temperatures and can be made of high strength steel to withstand very high pressure. Furthermore, in an internally heated system, the platinum electrical heating element and the insulation can withstand high temperature (up to .about.1500.degree. C.) and do not have to be stressed to contain the pressure.
The second general type of high pressure oxygen furnace system is the "externally heated system". In this system, the pressure vessel is inserted inside the furnace.
The main advantage of this design is that the pressure vessel is very small. This limits oxygen volume to a very small amount and minimizes the potential hazard. Also, the small size of the vessel makes it less expensive. Since the furnace heating element and insulation surround the pressure vessel, they are not exposed to high pressure or concentrated oxygen. This allows use of an economical, conventional, electrically heated furnace with base metal heating element.
This type of system, however, has two disadvantages. First, the furnace can only achieve limited temperatures and pressures because the vessel is heated and most metals normally used for vessels cannot withstand high pressure at a high temperature. Second, most metals burn with exposure to high pressure oxygen. This includes some of the strongest metals, such as titanium alloys, which react violently with high pressure oxygen and must be protected. Those metals that do not burn, such as the noble metals, platinum, gold, etc. become soft with exposure to high temperatures. Therefore they are unsuitable for pressure vessels. They are also very expensive.
Many metals and high strength alloys that do not burn or turn soft become embrittled upon exposure to high temperatures e.g., Haynes Alloy No. 214, (76% Ni, 16% Cr, 3% Fe, 4.5% Al, Y) becomes brittle because of formation of Ni.sub.3 Al gamma prime in the temperature range of 600.degree. C.-950.degree. C. (page 2, Haynes Alloy No. 214 booklet, Cabot Corporation, Kokomi, Ind. 46902).
It should be understood in these types of externally heated high pressure oxygen ovens, because the oven is being heated and cooled each time the furnace system is used, the pressure vessel is likely to embrittle and become more quickly liable to catastrophic failure.
Temperatures of over 900.degree. C. are necessary to make many superconductors. See Donald E. Morris, U.S. Pat. application Ser. No. 263,750, entitled Super Conductor, filed Oct. 28, 1988 and now abandoned. However, commercially available externally heated high pressure furnaces cannot reach the needed temperatures. They are mainly intended for inert atmospheres (as distinguished from oxygen), and are surely not rated to withstand the attack of hot concentrated (high pressure) oxygen.
A supplier of pressure vessels and reactors for use at high temperature and pressure is Leco Corporation, Tem-Pres Division, Bellefonte, Penna. 16823. They manufacture pressure vessels of materials identified as: 1) 316 Stainless Steel, 2) Unitemp L-605, 3) Rene and 4) Rene.sup.2. According to Leco, 316 Stainless Steel is rated for a maximum temperature of 550.degree. C., Unitemp L-605 and Rene are rated to a maximum of 750.degree. C. and Rene.sup.2 attains 900.degree. C. (Specification sheets LRA-488 and MRA-1085, Leco Corporation, Tem-Pres Division).
Another supplier of such apparatus is Parr Instruments, Moline, Ill. 61265. They produce pressure vessels of 1) Monel 400, 2) Inconel 600, 3) Hastelloy C-276, 4) Hastelloy B-2, 5) Titanium, 6) Nickel, 7) Zirconium, 8) Carpenter 20Cb-3, 9) C1018 carton steel, 10) 303 Stainless Steel and 11) 316 Stainless Steel (pages 8-10, Parr Instruments "Reactors and Pressure Vessels" catalog). According to page 11 of Parr Instruments catalog, the maximum temperature of any of these materials is only 600.degree. C., specified for 316 Stainless Steel, Inconel 600, and Hastelloy C-276. It is clear that all of these available pressure vessels are inadequate for synthesizing or heat treating superconducting materials at temperatures in the range of 900-1000.degree. C. at high pressures.